This invention relates to variable speed constant frequency electrical power systems and more particularly to such systems which include an auxiliary power output.
DC link variable speed constant frequency (VSCF) electrical power systems, such as those typically used in aircraft applications, include a generator which is driven at a variable speed to supply a DC voltage to an inverter by way of a pair of DC link conductors. The output voltage of the generator is controlled by a regulator that monitors the inverter output and adjusts the generator field current to regulate the DC link voltage. In some DC link VSCF aircraft applications, it becomes necessary to provide auxiliary power supplies such as 270 volts DC to energize control service actuators or 28 volts DC to energize avionic units. These auxiliary power supplies can be fed from the 115 volts AC inverter output. However, due to the vulnerability of these auxiliary supplies to an inverter failure, the system redundancy and flexibility are greatly reduced. Furthermore, during AC bus faults, the AC voltage may be reduced to zero.
Another approach, which bypasses the inverter and vulnerability problem, is to feed the auxiliary supplies directly from the DC link conductors, which receive power from the generator through a rectifier. Although the reduced voltage problem still remains, it is not as severe. For example, under AC bus fault conditions, the DC link voltage may be intentionally reduced to as little as 35% of its normal value in order to limit current, voltage and thermal stresses in the inverter. The size and weight of an auxiliary power supply would have to be considerably increased in order to regulate and maintain normal output voltage under such reduced input voltage conditions. It is therefore desirable to provide a means of compensating for the reduced input voltage to the auxiliary power supplies, without increasing their range of regulation, which would require an increase in size and weight.